Manufacturers in the tobacco industry are seeking to develop means of selective filtration in order to reduce the levels of certain constituents of main-stream cigarette smoke. For this purpose, various constructions of filter rods have been devised, involving in many cases the use of porous particles having adsorbent surfaces, particularly activated carbon particles. The inclusion of such particles in a filter rod can have a major impact on the efficiency of the filter but can involve more complicated and expensive filter rod manufacturing processes.
One approach has been to have a multi-section filter in which carbon particles are confined to an inner section of the filter, with the part of the filter which goes in the mouth being a standard cellulose acetate filament filter. In a triple-section filter, for example, the middle section may comprise a bed of loose carbon particles. However, a loose bed of particles in the cigarette filter may sometimes be by-passed as a filtration medium by channeling of the smoke stream passing through it.
Another approach has been to include carbon in filter rods by adhering the carbon particles to the filaments through use of plasticizers or adhesives sprayed onto the crimped tow at the filter rod making stage. This approach can however lead to variability in the application of the particles.
Traditionally, additional components for incorporation into cigarette filters have been added at or after the stage of forming filter rods from crimped tow. For example, U.S. Pat. No. 6,145,511 discloses the addition of various compounds to filters during conventional filter rod manufacture. It is stated that the additional components are mixed with the traditional plasticiser triacetin in the rod maker process.
The processes described above which involve the incorporation of additional components at the rod making stage add complexity to the rod making process. Furthermore, some processes are inflexible in that the rod making machine often needs to be custom designed or require extensive modification to perform the step of adding the particular substance in question.
Another approach to incorporating additional components is disclosed in WO 91/12737. This document discloses the step of dissolving acidic compounds into a cellulose acetate spinning solution prior to spinning filaments for use in the manufacture of a filter. The incorporation of acidic materials into the spinning solution is intended to alter the nicotine to tar ratio in filtered cigarette smoke.
The present invention seeks to provide an improved method for the manufacture of filter tow, which may be used in the manufacture of filter rods and cigarettes. The present invention also seeks to provide improved filter tow.